Illumination arrangement for a projection system

ABSTRACT

An illumination arrangement ( 1 ) for a projection system is proposed comprising a light source device ( 10 ) and a light collecting, integrating and redirecting device ( 20 ). The light source device ( 10 ) comprises at least one solid state light source device ( 30 ). The light collecting, integrating and redirecting device ( 20 ) comprises at least one light integrating device ( 50 ), which is adapted to directly receive and to integrate at least a part of primary illumination light (L 1 ) generated by and emitted from at least one associated of at least one solid state light source device ( 30 ).

The present invention relates to an illumination arrangement, inparticular for a projection system, or the like, and more particular toan illumination arrangement for a projection system which employs solidstate light sources.

Nowadays, in many electronic appliances display devices are necessaryfor displaying information to a user or an audience. Because of thelarge variety of different types of electronic appliances having such adisplay device it became necessary to develop display devices for whichonly a limited space and/or a limited power consumption are available.Therefore, the idea of involving an array of light emitting diodes (LED)came up, but it was realized at the same time that known and state ofthe art light emitting diodes have only a very limited light outputcapability.

It turns out that in recent years, a lot of efforts have been done toprovide illumination arrangements allowing an uniform illumination of asurface (e.g. a μ-display) showing a high compactness.

An example of such an illumination arrangement will be explained indetail while making reference to FIG. 21. An illumination arrangement 1′comprises a polychromatic light source array 2′, a pyramidal light pipe3′, and a target surface 4′ to be illuminated. The light source array 2′comprises a first to fifth light source 2′₁ to 2′₅ which preferably mayemit light of different colors, respectively. The light pipe 3′ fulfillsthe following functions: Collection and collimation of the light beingemitted from the polychromatic light source array 2′ and homogenizationof the illumination of the target surface 4′. Thus, a polychromatic,uniform and collimated illumination of the target surface 4′ can beachieved.

The advantage of the pyramidal light pipe 3′ is that the dimensions ofan overall light emitting surface of said light source array 2′ (whichis indicated by the parameter S1, the area of said surface) issufficient for illuminating the relatively large target surface 4′(which dimensions are indicated by the parameter S2, the area of saidsurface 4′). However, there is the problem that the size of an inputsurface 5′ of the light pipe 3′ limits S1 and therefore limits themaximal number of light sources of 2′₁ to 2′₅. As a consequence, thebrightness of the light source array 2′ and thus the brightness of thelight illuminating the target surface 4 is limited.

It is an object of the present invention to provide illuminationarrangements, in particular for a projection system, which are capableof using solid state light source devices having only low light outputcapabilities and/or which—at the same time—enable an easy and reliableoptical coupling of the primary illumination light from the solid statelight source device to projection optics.

The object is achieved by illumination arrangements according to thefeatures of independent claims. Preferred embodiments of the inventiveillumination arrangements are within the scope of the respectivedependent sub-claims.

-   -   In the following a first solution of the object of the present        invention is described:

The illumination arrangement according to the first solution of theobject of the present invention is adapted for a projection system, orthe like, and comprises a light source device and a light collecting,integrating and redirecting device. The light source device is adaptedfor generating and for emitting primary illumination light. The lightcollecting, integrating and redirecting device is adapted for receivingat least a part of said primary illumination light from said lightsource device in a direct manner. The light collecting, integrating andredirecting device is further adapted to redirect said received primaryillumination light so as to obtain directed primary illumination light.Additionally, said light collecting, integrating and redirecting deviceis adapted for outputting said redirected primary illumination light ora derivative thereof as secondary illumination light. According to thepresent invention, said light source device is or at least comprises atleast one solid state light source device. Said light collecting,integrating and redirecting device comprises at least one light valvedevice which is adapted for receiving said redirected primaryillumination light and for outputting said secondary illumination lightin a controllable manner. Further, said light collecting, integratingand redirecting device comprises at least one light integrating devicebeing adapted for directly receiving and for integrating at least a partof said primary illumination light generated by and emitted from atleast one associated of said at least one solid state light sourcedevices and for outputting said redirected primary illumination light ora derivative thereof.

It is therefore a basic idea of the present invention to use at leastone solid state light source device as said light source device. It is afurther basic idea of the present invention to have at least one lightintegrating device which is adapted for directly receiving and forintegrating at least a part of said primary illumination light.

There are several possibilities of realizations for said solid statelight source devices. First of all, it is preferred to have the solidstate light source device comprised of a single or of a plurality ofsolid state light sources.

If a plurality of solid state light sources is involved said pluralitymay be built-up by or may comprise an array of solid state lightsources.

It is of particular advantage to involve different lands of solid statelight sources, in particular if each of which is adapted for generatingand for emitting radiation or light of distinct spectral ranges orcolours. In this case, they may be in particular organized in distinctgroups, wherein in particular each group is then capable of producingradiation or light of a given spectral range or colour.

According to a further advantageous embodiment of the inventiveillumination arrangement each of said solid state light sources is asingle light emitting diode (LED) or a multiplicity of light emittingdiodes. Also edge-emitting LEDs (EELED) or pluralities thereof can beused.

Alternatively or additionally, each of said solid state light sources isa single vertical cavity surface emitting laser device (VCSEL) and/or alaser diode (LD) or a multiplicity of vertical cavity surface emittinglaser devices and/or laser diodes (LD).

Alternatively or additionally, each of said solid state light sources isa single resonant cavity light emitting diode (RCLED) or a plurality ofresonant cavity light-emitting diodes.

To allow most of the primary illumination light generated and emitted bythe light source device to be used and to be optically coupled toprojection optics and to avoid primary illumination light to escape fromthe location of its generation without being collected it is alsoproposed that said light integrating device and said at least oneassociated solid state light source device are disposed in closedspatial proximity or relationship to each other.

It is in particular suggested that said light integrating device andsaid at least one associated solid state light source device aredisposed in direct mechanical contact to each other.

In contrast, the collecting property of the light integrating device canbe increased if according to a further preferred embodiment said lightintegrating device and said at least one associated solid state lightsource device are adapted to have a gap structure between them, inparticular an air gap, an evacuated gap, the gap width of which beingsmall in particular compared to the cross-sections of the lightintegrating device and/or said at least one associated solid statelight-source device.

According to this measure and in accordance to the TIR or total internalreflection condition even more light generated and emitted by theassociated solid state light source device can be collected andintegrated by said light integrating device, if no air gap is present.

To further increase the light transmission from the associated solidstate light source devices to the associated light integrating devicesit is proposed in accordance to a further preferred embodiment of thepresent invention that said light integrating device has a lightincidence aperture, that said associated solid state light source devicehas a light emitting aperture, and that said light emitting aperture isless than or equal to said light incidence aperture with respect totheir diameter or cross-section area. According to this particularmeasure the cross-section or the area of the light incidence aperture ofthe light integrating device gets the best illumination with respect tothe primary illumination light generated by and emitting from theassociated solid state light source device.

There are different possibilities of building-up said light integratingdevice. First of all said light integrating device may be a light pipe,an integrator rod, and/or the like.

Said light integrating device may be a solid rod, made in particular ofplastic, glass, or another optical transparent material.

Alternatively, said light integrating device may be built-up as a hollowtube device or tube element having reflecting or mirrored inner walls orside faces.

According to the above-mentioned measures, said light integrating deviceacts as a light guide for the received primary illumination light.

Advantageously, said light integrating device has a square, rectangular,hexagonal or—in particular equilateral—triangular cross-section toobtain a uniform distribution. Oval or circular cross-sections are alsopossible if there are uniformity restrictions possible.

Additionally or alternatively, said light integrating device may bebuilt-up as a light mixing device, in particular as a beam splitterdevice, a colour cube device, and/or the like.

In this case said light integrating device may have a plurality of lightincidence apertures and at least one light output aperture.

According to this measure it is possible to use said light integratingdevice as an input stage for the primary illumination light of differentand separated solid state light source devices, the primary illuminationlight of which entering different light incidence apertures and themixing light leaving the light integration device after being mixedwithin said light integrating device and exiting the light integratingdevice from said light output aperture.

Of course, different light integrating devices can be combined with eachother so as to combine and integrate and redirect primary illuminationlight stemming from different and spatially separated different solidstate light source devices to yield a secondary illumination lighthaving best illumination and projection properties.

According to a further aspect of the present invention a light couplingand/or guiding improving arrangement is provided which is adapted and/orarranged so as to improve the coupling and/or the process of guiding ofsaid primary illumination light from said light source device to and/orwithin said light collecting, integrating and re-directing device and inparticular to and/or within said light integrating device.

-   -   In the following a second solution of the object of the present        invention is described:

According to the second solution of the object of the present invention,an illumination arrangement according to a first solution of the objectcomprises at least two light sources for generating first light beams,respectively, a light mixing device for inputting said first light beamsand combining them to a single second light beam, and a pyramidal lightpipe which inputs the second light beam and outputs a third light beam.The third light beam is the desired output light beam which illuminatesthe target surface (which may for example be a “μ-display”).

Preferably, three light sources are used, each of them generating afirst light beam of one of the colors green, red, and blue,respectively. However, also other colors may be used. Further, more thanthree light sources may be used.

In a preferred embodiment, the light mixing device is a color cube whichshows at least two input surfaces for inputting one of the first lightbeams, respectively, and one output surface for outputting the secondlight beam. Alternatively, the light mixing device is constituted by adichroic filter or a combination of dichroic filters. Each of thedichroic filters shows at least one input surface for inputting one ofthe first light beams, respectively, and one output surface. Thecombination of dichroic filters may, for example, comprise a first and asecond dichroic filter, wherein the first dichroic filter shows twoinput surfaces for inputting two of the first light beams, and oneoutput surface for outputting a first combined light beam, wherein thesecond dichroic filter comprises one input surface for inputting one ofthe first light beams, one input surface for inputting the firstcombined light beam, and one output surface for outputting a secondcombined light beam which is the single second light beam mentionedabove.

The terms “combining/combine” in the expressions “combining the firstlight beams to a single second light beam” and “a first combined lightbeam” and “a second combined light beam” may mean superposition of lightbeams of a common cross-section, or a light beam which alternatelyconsists of different single light beams over the time (i.e. nosuperposition, known as “sequential coloring”).

Advantageously, the length and the width of output surfaces of the lightsources are equal to or less than that of respective input surfaces ofthe color cube/the respective input surface of the dichroic filters.Thus, an optimum of light power is inputted into an input surface of thelight pipe. However, the length and width of output surfaces may also besmaller than the input surfaces of the color cube/dichroic filters.Preferably, the length and the width of an output surface of the colorcube/dichroic filter which outputs the second light beam is equal to orless than the length and the width of an input surface of the pyramidallight pipe which inputs the second light beam.

Between the light sources and the light mixing device and/or between thelight mixing device and the pyramidal light pipe air gaps may beprovided, respectively, in order to improve light collection.

Between each light source and the corresponding input surfaces of thecolor cube/dichroic filters additional pyramidal light pipes may belocated. This enables a great flexibility as far as the dimensions ofthe light mixing device are concerned. Preferably, in this embodiment,the lengths and the widths of the output surfaces of the light sourcesare equal to or less than the lengths and the widths of input surfacesof the additional pyramidal light pipes, and the lengths and the widthsof output surfaces of the additional pyramidal light pipes are equal toor less than the respective input surfaces of the color cube/dichroicfilters. The output surfaces of the light sources may, however, also besmaller than the input surfaces of the color cube/dichroic filters. Toincrease light collection, the additional pyramidal light pipes and thelight sources are in direct mechanical contact with each other. Tofurther improve light collection, between the additional pyramidal lightpipes and the light mixing device air gaps are provided.

As has become apparent, an important aspect of the inventiveillumination arrangement described above is that the use of a lightmixing device makes it possible to increase the output surfaces of thelight sources without increasing the input surface of the pyramidallight pipe. As a consequence, the brightness of a target service to beilluminated by the illumination arrangement can be increased while atthe same time the dimensions of the illumination arrangement areincreased only very slightly.

-   -   In the following a third solution of the object of the present        invention is described:

According to said third solution of the object of the present inventionan illumination arrangement is provided comprising at least two lightsources for generating first light beams, respectively, a light mixingdevice for inputting the first light beams and combining them to asingle output light beam, and at least two pyramidal light pipes whichare located between a light source and the light mixing device,respectively.

Preferably, input surfaces of the pyramidal light pipes and thecorresponding light sources are in direct mechanical contact with eachother. Further, between the pyramidal light pipes and the mixing deviceair gaps may be provided.

This embodiment also shows above mentioned advantages of a highcompactness and an increased brightness of an output light beam. Anadditional advantage of this embodiment is that the mixing device showslarger dimensions compared to that of the first embodiment. As aconsequence, the light mixing device of this embodiment is easier tomanufacture.

The colour cube and/or the dichroic filters preferably comprise at leasttwo glass prisms, respectively.

Advantageously, but not necessarily, between two of the glass prisms ofa color cube/dichroic filter a glass plate is provided, respectivelywhich is coated by a transmissive/reflective film, wherein the surfacedimensions of the glass plate are bigger than the surface dimensions ofsurfaces of the prisms sandwiching the glass plate.

-   -   In the following a fourth solution of the object of the present        invention will be described:

According to said fourth solution of the object of the invention anillumination arrangement is provided, in particular for a projectionsystem or the like. Said illumination arrangement comprises a lightsource device which is adapted for generating and for emitting primaryillumination light. Further a light collecting, integrating andre-directing device is provided being adapted for receiving at least apart of said primary illumination light from said light source deviceand for re-directing said received primary illumination light so as toobtain re-directed primary illumination light and for outputting saidre-directed primary illumination light or a derivative thereof assecondary illumination light. Said light collecting, integrating andre-directing device comprises at least one light integrating devicebeing adapted for directly receiving and for integrating at least a partof said primary illumination light generated by and emitted from atleast one associated light source device and for outputting saidre-directed primary illumination light or a derivative thereof.According to that particular solution a light coupling and/or guidingimproving arrangement is provided which is adapted and/or arranged so asto improve coupling and/or guiding of said primary illumination lightfrom said light source device to and/or within said light collecting,integrating and re-directing device and in particular to and/or withinsaid light integrating device.

It is therefore a basic idea of said fourth solution to provide a lightcoupling and/or guiding improving arrangement. This particular lightcoupling and/or guiding improving arrangement is adapted to improve thelight coupling from the light source device or a mixing device to saidlight collecting, integrating and re-directing device in particular tosaid light integrating device. Alternatively or additionally, said lightcoupling and/or guiding improving arrangement is adapted to improve theefficiency of the guiding process within said light collecting,integrating and re-directing device in particular within said lightintegrating device, for instance by reducing the escape of light fromthe light collecting, integrating and re-directing device and inparticular from said light integrating device.

According to a preferred embodiment of this fourth solution said lightintegrating device is or comprises a plain light pipe—in particular asolid integration rod—having a light incidence aperture and a side wall.Said side wall of said light integrating device is provided withreflecting means as said light coupling and/or guiding improvingarrangement or as a part thereof at its outer periphery at least in aneighborhood of said light incidence aperture. Further, said lightreflecting means is adapted and/or arranged to reflect light escapingfrom said light integrating device with a side wall thereof back intosaid light integrating device.

According to a further aspect of the present invention in the casewherein said light integrating device is or comprises a plain lightpipe, said light incidence aperture of said light integrating device maybe positioned in a neighborhood of a light exit aperture of said lightsource device and/or of said light mixing devices. In this case, betweensaid light incidence aperture of said light integration device in saidlight exit aperture of said light source device and/or said light mixingdevices refraction index matching means may be provided, in particularfilling a gap or a gap structure between said light incidence apertureof said light integration device in said light exit aperture of saidlight source device and/or of said light mixing devices.

Additionally or alternatively said reflection index matching means maybe a liquid, gel, and/or a glue.

Additionally or alternatively said refraction index matching means mayhave a refraction index which essentially coincides with the refractionindex of the material of the light integration device or with therefraction index of a material of the light source devices periphery.Said refraction index of said refraction index matching means may alsointerpolate between these values for refraction indices.

According to a further alternative or additional embodiment of thepresent invention said light integration device may or may comprise ahollow light pipe having a light incidence aperture. In this case saidlight incidence aperture of said light integrating device may bepositioned in a neighborhood of a light exit aperture of said lightsource device and/or of said light mixing devices. In this case asection or an enter section of the light integration device in theneighborhood of said light incidence aperture and/or being terminated bysaid light incidence aperture may be—in particular completely—filledwith a plain pipe section as said light collecting and/or guidingimproving arrangement or as a part thereof, in particular for matchingthe respective refraction indices.

-   -   In the following these and further aspects of the present        invention will be more elucidated:

Solid state light sources (SSLS) present a number of advantages forrear- and front-projector engines when compared with traditionally usedhigh pressure lamps. In particular, SSLSs allow colour management at thesource level; they allow a better colour saturation, and they have amuch longer lifetime. Moreover, SSLSs allow the design of new lightengine architectures leading to more compact and potentially cheaperdevices.

The improvement of the lumen output, e.g. of light emitting diodes(LEDs), make them a natural tentative candidate to be used inprojectors. As the light emitted by a single LED is not sufficient forsome projector applications, the idea of collecting the light emitted byan array of LEDs and in redirecting it through the light valve came up.If the LED array covers a surface greater than the panel surface, lightpipes or optical fibres commonly are used to collect the light of eachindividual LED. This approach requires a precise and costly assembly.

Instead, it is proposed to optimise the design of the illuminationengine based on today and incoming solid state light source technologybased e.g. on optimised light source configurations combined with anintegration rod.

The problem of common projectors using solid state light sources is thelimited brightness or lumen output reaching the screen. The brightnessdepends on the source throughput, the directivity of emission of thesource and the optical efficiency of the projector engine.

As types of solid state light sources light emitting diodes (LED),edge-emitting light-emitting diodes (EELED) resonant cavity lightemitting diodes (RCLED), laser diodes (LD), and vertical cavity surfaceemitting lasers VCSEL are suggested.

The limitations of available LEDs are the limited throughput and thenon-directive emission according to the Lambertian emission law.

VCSEL and RCLED have the big advantage to have a very directiveemission. The limitations of available VCSEL and RCLED are e.g. thatcommercially available VCSEL and RCLED and lab samples of visible VCSELand RCLED are not powerful enough, and are only available for red andblue.

LD's have the big advantage of a very directive emission. The limitationof available LDs is that commercially available LDs are powerful enoughin red only.

The invention proposes e.g. projector engine designs which optimise theuse of LEDs of today technology as well as the use of VCSELs and RCLEDsas well as LDs. Moreover, some of the proposed designs are extremelycompact and will allow the realization of embedded projectors, which isimpossible with today technology.

A first proposed approach consists in using LED arrays whose overallsurface is smaller or equal to that of the light valve. In this case thelight is guided onto the light valve by a single and simple light pipe,whose role also consists in making uniform the light distribution asshown in FIG. 1. The light pipe, also called integrator rod, can eitherbe a solid glass or plastic rod or a hollow mirrored tube e.g. with arectangular cross section. For a good coupling efficiency, an air gap isrequired between the LED array and the light pipe, and between the lightpipe and the light valve. To avoid that too much light escapeslaterally, this air gap should be kept as small as possible. Thesimplicity of the design is based on the matching of the light pipe andthe light valve cross sections. Thus the light valve is uniformlyilluminated by light coming out from the light pipe. The cross sectionof the LED array has to be smaller or equal to the cross section of thelight valve.

Beside marginal losses produced by Fresnel reflection at the light pipeextremities, all the light emitted by the LED array is directed onto thelight valve. At this point, the light has to go through the light valveand the projection optics before reaching the projection screen. Boththe light valve and the projection optics have a limited angle ofacceptance or aperture. This means that only the light included within agiven cone of acceptance is going to reach the screen, the rest beinglost.

The aperture of the projector is determined by the F/number of theprojection optics, typically between F/3 (half cone angle of 10°) andF/2 (half cone angle of 15°). This means that at the light valve plane,all the light which is not within the cone of acceptance is lost.

LEDs, without complementary optics, emit light vs. a Lambertiandistribution. When the light reaches the projection optics, only theproportion of light within the cone of acceptance is going to reach thescreen. As shown in FIG. 3, only small part of the emitted energy (3.0%for F/3 and 6.7% for F/2) is included within the acceptance cone.

The optics between the LED array and the light valve can redirect therays within the cone of acceptance, increasing the efficiency of thedevice. This can be done either, by using collimation micro-lenses infront of the LED array as shown in FIG. 4A, or by using a pyramidalintegration rod according to FIG. 4B. In both cases however, the activeemitting surface is smaller than the light valve surface. In otherwords, the efficiency of the illumination engine can be improved byusing collimation optics, but at the cost of the light throughput of theLED array because of a diminution of the emitting surface.

Overall the limited (but improving) throughput of the Lambertianemitting LED arrays limit their use to projectors having low lumenoutput requirements, e.g. in rear-projection TV.

Vertical cavity surface emitting lasers CVCSEL) have the interestingproperty that they emit light within a cone of typically beam divergenceof ±8° which is smaller than the cone of acceptance of the projectionoptics. Therefore, beside the losses of each individual optical element,all the energy emitted by a VCSEL array would reach the screen. As shownin FIG. 2, the VCSEL array can have the exact cross-section of the lightvalve. The intermediate optics, i.e. the integration rod, is only usedas light distribution uniformiser and has no collimation functionality.

Laser diodes have astigmatic emission, i.e. they do not need to becollimated in one axis (like VCSELs), but require collimation in theother axis. Asymmetric light pipes are therefore used.

FIG. 5 illustrates the architecture of a three-colour-path transmissiveprojector based on solid state light sources either LED array of VCSELarray and/or RCLED array. The imaging optics light valve and projectionlens can be the same as those of a high-pressure lamp projector.However, the illumination engine is simplified and more compact, nofly-eye lens, no relay lenses are involved.

Solid state light sources can also be used in sequential colourprojectors. The advantage over HP lamp sequential projectors is thatcolour management can be done directly at the source level, i.e. nocolour filter for colour separation is needed. The colour selection ismade electronically by switching on and off the different light sources.

Very compact architectures can be achieved when using back-lightingillumination light pipes. The illumination light pipes have a similardesign to those used for the back-lighting of T-LCD displays, typicallyused in laptops an cell phones. The light is guided inside the lightpipe by total internal reflection and is selectively out-coupled fromthe light pipe by scattering zones placed along the light pipe surface.These compact projectors can be embedded into portable devices such asUMTS cell phones, camcorders, palmtops, or the like.

What distinguish configurations of FIGS. 5, 7 and 8 is the placeavailable for the sources, i.e. for the emission surface, and thereforethe resulting lumen throughput of the projector. The extremely reducedspace available for the sources in FIG. 8 configurations implies the useof a highly efficient light sources, like VCSEL arrays and/or RCLEDarrays.

Configurations based on reflective light valves can also be built aroundsolid state light sources. Nevertheless, the integration rod cannot beplaced in close contact with the light valve, as the light should escapethrough the projection lens. In other words, some kind of beam splitteris needed in front of the light valve. The uniform distribution of lightcoming out of the integration rod has to be projected by some relayoptics on the light valve. Basically all the standard reflectiveprojector configurations based on integration rods can be adapted; inorder to use solid state light sources.

These reflective configurations are not as compact as the proposalsbased on transmission light valves. On the other hand there is space toplace some kind of polarization recycler between the integration rod andthe light valve. The same remark applies to traditional transmissionconfigurations which make use of integration rod, relay optics andpolarization recycler. They can also be adapted in order to make use ofsolid state light sources.

When compared with traditional high pressure lamp projectors theinvention offers the following advantages:

-   -   Better colour saturation and larger colour gamut    -   Colour management at the source level        -   no need of colour filters        -   electronic sequential colour management        -   possibility of dynamic contrast adjustment.    -   Much increased lifetime of the source    -   No infrared emission on the optical path (cold light source)        -   possibility to use low cost plastic optics    -   Possibility to improve the red channel of current        three-colour-path projectors.

When compared with other proposed LED projectors the invention offersthe following advantages:

-   -   Simpler design based on integration rod in close contact with        the source panel and the light valve (no fly-eye lens, no relay        optics)    -   Optimised LED-to-light-valve coupling efficiency    -   Use of directive emitting VCSELs, RCLEDs, or LDs for much        increased optical efficiency    -   Ultra-compact configurations based on back-lighting light pipes.

In the following, some further general and theoretical aspects of theinventive concept and its realisations are given taking reference toFIGS. 10 to 12:

One Aspect of the present invention and its embodiments is to solve theproblem of finding a configuration which maximizes the illumination of asurface using an array of LEDs. Moreover, the illumination of thesurface needs to be uniform and the direction of the rays kept within alimited aperture. The aperture or the angle of acceptance is determinedby the numerical aperture of the imaging optics. The difficulty of thetask comes from the fact that the angle of acceptance is generally smallwhen compared to the large angular emission of the LEDs, having e.g.typically a Lambertian distribution.

An illuminated surface may be, for example, the probe plane of amicroscope, or the light valve plane of a projector. The imaging opticsare in these cases the microscope objective or the projector objective.

A goal consists of illuminating a plane uniformly with maximum lightpower and within a limited aperture of the optics. In other words,

-   -   1. maximization of the collection efficiency or the capture of        light emitted by the LEDs,    -   2. maximization of the collimation efficiency or the directing        of light with the aperture of the given optics, and    -   3. maximization of the light engine efficiency or the        minimization of the optical losses of the components        have to be achieved and are achieved by the present invention.

The problem which is illustrated in FIG. 10, consists of finding aconfiguration which maximizes the illumination of a surface using ansolid state light sources or array 33 of LEDs 31. Moreover theillumination has to be uniform and the direction of the rays kept withina limited aperture.

The illuminated surface S₂ may be, for example, the probe plane of amicroscope, or the plane of a light valve 40 of a projector. The maximalaperture of the illumination rays is then defined by the numericalaperture of the microscope objective or the F-number of the projectorobjective 70.

The difficulty of the task stems inter alia from the non-directiveradiation pattern emitted by the LEDs 31, which is e.g. typically aLambertian distribution. This light needs to be redirected onto thelimited surface to illuminate and within the limited aperture of theoptics.

Theoretical Background

In the following, again reference is taken to FIGS. 10 to 12.

All illumination designs have to take into account the étendue theoremwhich states that the étendue or optical extent along an optical systemcannot be reduced. For a given surface S the étendue E is defined by thesurface S multiplied by the solid angle Ω sustaining the light rays,i.e.E=S·Ω,  (1)according to FIG. 11 for Definition of the étendue E.

In the general system illustrated in FIG. 10, the maximal useful étendueE₂ is defined by the surface S₂ to illuminate and the solid angle Ω₂.For instance, if for the étendue E₁ of the source the relation E₁>E₂holds, then part of the light is lost.

The solid angle Ω₂ is function of the aperture of the optics and isgiven by the equationΩ₂=2π·(1−cos φ₂)=4π·sin²(χ₂/2),  (2)where φ₂ is the half angle of the cone of aperture.

The étendue E₁ of the LED array is defined as

$\begin{matrix}{{E_{1} = {{\Omega_{1} \cdot {\sum\limits_{i = 1}^{N}S_{1i}}} = {2{\pi \cdot S_{1}}}}},} & (3)\end{matrix}$where S₁₁ is the emission surface of each individual LED, N is thenumber of LEDs in the array, and 2π is the solid angle of the hemispherecorresponding to the Lambertian emission.100% Collimation Efficiency System

The étendue theorem states that the étendue along an optical systemcannot be reduced. Therefore, in order to achieve an optical system witha 100% collimation efficiency, the emission surface S₁ of the LED arraycannot exceed S_(1max) as is shown by the following relations

$\begin{matrix}\begin{matrix}{{E_{1} \leq E_{2}},} \\{{{2{\pi \cdot S_{1}}} \leq {4{\pi \cdot {\sin^{2}\left( \frac{\varphi_{2}}{2} \right)} \cdot S_{2}}}},{and}} \\{{S_{1} \leq S_{1\mspace{11mu}\max}} = {2 \cdot {\sin^{2}\left( \frac{\varphi_{2}}{2} \right)} \cdot {S_{2}.}}}\end{matrix} & (4)\end{matrix}$System with Limited Collimation Efficiency

If for the surface of emission the relation S₁≧S_(1max) holds, part ofthe emitted light will not reach the surface S₂ within the aperture φ₂,and will therefore be lost.

The problem is analysed by looking at what is the emitted cone orhalf-angle φ₁ at the surface S₁ which is within the aperture of theoptics or half-angle φ₂ at the surface S₁.

From the étendue theorem it follows that

${{S_{1} \cdot 4}{\pi \cdot {\sin^{2}\left( \frac{\varphi_{1}}{2} \right)}}} = {{S_{2} \cdot 4}{\pi \cdot {{\sin^{2}\left( \frac{\varphi_{2}}{2} \right)}.}}}$is fulfilled. Therefore,

$\begin{matrix}{\varphi_{1} = {{2 \cdot \sin^{- 1}}\left\lfloor {\sqrt{\frac{S_{2}}{S_{1}}} \cdot {\sin\left( \frac{\varphi_{2}}{2} \right)}} \right\rfloor}} & (5)\end{matrix}$is also fulfilled.

The coupling efficiency η_(c) is defined as the ratio of the emittedenergy W₁ within the cone defined by φ₁, and the total energy W emittedby the source or surface S₁, i.e.:

$\begin{matrix}{\eta_{c} = {\frac{W_{1}}{W}.}} & (6)\end{matrix}$

In the case of a Lambertian light source with an emission angle γ=π/2,the coupling efficiency becomes

$\begin{matrix}{\eta_{c} = {\frac{\int_{- \varphi_{1}}^{+ \varphi_{1}}{\int_{- \varphi_{1}}^{+ \varphi_{1}}{{\cos(\alpha)}{{\cos(\beta)} \cdot {\mathbb{d}\alpha}\; \cdot {\mathbb{d}\beta}}}}}{\int_{{- \pi}/2}^{{+ \pi}/2}{\int_{{- \pi}/2}^{{+ \pi}/2}{{\cos(\alpha)}{{\cos(\beta)} \cdot {\mathbb{d}\alpha} \cdot {\mathbb{d}\beta}}}}} = {\sin^{- 2}{\varphi_{1}.}}}} & (7)\end{matrix}$

The luminous flux W₂ reaching the surface S₂, within the aperture φ2 isproportional to the emission surface S₁ of the source and to thecollimation efficiency η_(c),W ₂∝η_(c) ·S ₁.  (8)

Three cases can be distinguished:

-   1) S₁≦S_(1max) and η_(c)=1, all the light emitted by the source can    be used: W₂∝S₁,-   2) S₂≦S₁>S_(1max) and η_(c)<1, part of the light is lost, but as the    surface of emission S₁ increases, W₂ increases, and-   3) S₂>S₁, the surface of emission S₁ increases, but W₂ does not    increase.    Proposed Solution and Features

Different solutions based on reflectors and/or refractive lenses havebeen proposed for the collimation of LEDs. The drawback of these knownapproaches is that it is difficult to collect 100% of the light in thedesired direction. Moreover the optics surrounding the LED iscumbersome, artificially increasing the étendue of the source. Inaddition, further optics is needed to make the illumination uniform,e.g. fly-eye lenses or an integration rod.

According to the present invention an approach based on—in particularpyramidal shaped—integration rods is proposed. This approach fulfils thethree needed functions of

-   -   collecting the light emitted by the LED array,    -   collimating within the aperture of the optics, and    -   homogenising the illumination.

The working principle of a pyramidal integration rod or PIR isillustrated in FIG. 5. The PIR has an entry surface S′, an exit surfaceS″, and length L. The PIR can be an empty tube whose internal faces aremirrors, or a plain transparent material—e.g. mineral glass, plastic orthe like—of index n. For a plain PIR, the rays are reflected on thesurface by total internal reflection or TIR. As is illustrated for tworays in FIG. 5, the angle with respect to the PIR surfaces normal issmaller at the exit of the pipe than at its entrance. Given the étenduetheorem, the collimation is defined as

$\begin{matrix}{{\Omega^{''} = {\frac{S^{\prime}}{S^{''}} \cdot \Omega^{\prime}}},} & (9)\end{matrix}$where Ω′ is the solid angle of the ray before the PIR, and Ω″ is thesolid angle of the ray after the PIR. The relations S″>S′ and Ω″<Ω′ arefulfilled.

As for a normal integration rod, the rays are mixed within the rod. Twocondition have to be fulfilled in order to get an uniform distributionat the PIR exit surface:

-   1. The PIR cross-section has either to be square, rectangular, (in    particular equilateral) triangular, or hexagonal.-   2. The PIR has to be long enough to allow enough reflections against    the PIR surface.

The theoretical collimation efficiency η_(c) is achieved for L≧L_(c).Above the length L_(c) the collimation efficiency is constant. L_(c) isdetermined experimentally or by ray-tracing simulation, in a case bycase basis.

In problem described above, the PIR entry surface S′ has to coincidewith the LED emission surface S₁, and the PIR exit surface S″ has tocoincide with the surface S₂ to illuminate. As the LED array isconstituted by a set small emission surfaces S₁₁, a micro PIR can beplaced in front of each LED. The light is then collected by a bigger PIRor integration rod. The three systems illustrated in FIG. 6 are allequivalent, given the length of the PIR is long enough to complete thecollimation and the homogenisation.

Main advantageous features of the present invention are the usage of asingle optical element is used for light collection, light collimation,and light homogenisation. By using a single component from the lightsource (LED array) to the illuminated plane, the proposed approachminimizing the optical loses, lowers the manufacturing costs, andsimplifies the device assembly.

-   -   These and further aspects will also be elucidated in the        following:

The invention proposes inter alpha an illumination scheme based on acolour multiplexer and light pipes. It allows the uniform illuminationof a surface (e.g. a m-display) by the combining the light of differentcolour light sources (e.g. red, green, and blue LEDs). The inventionconsists in the combination and particular assembly of the differentoptical components allowing an extremely compact embodiment.

Colour combination can either be achieved by using a suite of dichroicfilters or by using a colour cube. The combination of dichroic filters(coated glass plates) allows the combination of different colour beamsinto a single polychromatic beam. The coatings reflect one single colour(e.g. green or red) and transmit all the others. Note that the suite ofdichroic filters can also be applied on prisms.

The colour cube is formed by the assembly of four prisms. The prismsurfaces forming the cube diagonals are coated in such a way to reflectone colour (e.g. red or blue) and to transmit all other colours. In thisway, three different colour beam can be recombined in a singlepolychromatic output beam.

It should be noted that these colour combination schemes do not fulfilany function in order to homogenize the illumination produced by theoutput beam.

A illumination engine has been developed using a pyramidal light pipe.The light pipe fulfils the following functions: collection of the lightcoming from the light source (or array of light sources), collimation ofthe light coming from the source(s), and homogenisation of theillumination. It should be noted, that an array of different coloursources can be used. In this way a polychromatic, uniform, andcollimated illumination is achieved.

However, the limited surface of the light pipe input surface determinesthe maximal number of source elements, and therefore limits thebrightness of the source

A problem consists in finding a configuration which has the samefunctionality of the illumination set-up presented with lightcollection, light collimation, homogenisation of illumination, but witha increased surface available for the sources (allowing an increasedbrightness.

A key parameter (requirement) is the compactness of the embodiment (e.g.for illumination of m-displays).

A basic idea in order to increase the brightness of the colourillumination device is to combine the properties of the colourcombination schemes described, in particular with the 2^(nd) proposalbelow, and the light pipe illuminator described, in particular in the3^(rd) proposal below. One goal is to have a bigger surface for couplingthe light sources (e.g. LEDs) and to achieve a uniform and collimatedillumination with a pyramidal light pipe.

1^(st) Proposal:

The first proposed set-up, consists of a colour cube for collecting andmultiplexing the light of the single colour-light sources, and a pyramidlight pipe for the homogenisation and collimation of the illumination,see FIG. 13.

Note that an air gap is required between the light sources and thecolour cube as well as between the colour cube and the light pipe. Theair gap reflects by TIR (total internal reflection) the rays which wouldotherwise escape the cube. These rays would either be absorbed orpropagate in an undesired direction, producing optical losses. Note thatsome rays are practically unaffected by the air gap

2^(nd) Proposal:

The second proposal uses a sequence of dichroic filters to combine thedifferent colour sources, see FIG. 14.

It should be noted that the dichroic filters are in glass cubes, calledhereafter dichroic cubes. The light sources, the dichroic cubes, and thelight pipe are separated by an air gap. As for the 1st proposal, the airgap allows to guide ray by TIR and minimize optical losses. The use ofdichroic filter on glass plates would also let rays escape in undesireddirections producing optical losses.

The 2^(nd) proposal is less compact than the 1st proposal, but it issimpler technologically speaking. It is therefore cheaper to realize, inparticular for very compact dimensions.

3^(rd) Proposal:

For compact configuration, the first two proposals may be difficult tomanufacture. For example, in the case of the illumination of a 0.7″ LDCm-display, the cube edge has a typical size between 2.0 mm and 4.0 mm.

A way to relax the cube dimension constraint is shown in FIGS. 15 and16. The two configurations are functionally equivalent. However thepositioning of the cube in an intermediate position is compacter

It should be noted that the cube is surrounded by air gaps in order tominimize optical loses, as already explained. However, when the sourceis directly in front of the light pipe, there is no need to include anair gap between the source and the light pipe. Actually when usingsurface mounted LEDs, the light extraction efficiency of the source isincreased when the pipe is in contact with the LED surface. This resultsin significant increase of the overall optical efficiency.

The semiconductor active surface emits rays in random directions. Due tothe high refractive index of the semiconductor when compared to therefractive index of the epoxy layer, part of the rays are trapped byTIR. The same happens for part of the rays between the epoxy and the airgap. However when the epoxy is in contact with the light pipe, all therays leaving the epoxy are coupled into the light pipe, as therefractive index of the epoxy and the refractive index of the pipe canbe (need to be) chosen close to each other (close to index matchingcondition).

Reminder: Total Internal Reflection Condition

For a ray in a high refractive index medium reaching the interface witha lower refractive index medium, if the angle of incidence is largerthan the critical angle, then the ray is totally internal reflected.Otherwise the ray is refracted and propagates in the low refractiveindex medium, see FIG. 26.

Prism Coating:

When using colour or dichroic cubes for the proposed illuminationconfigurations, the full dichroic surface is used. In other words, thefull surface should be coated by the dichroic filter. In practicehowever, the coating does not stick to the surface close to the edges(see FIG. 12). When the prism size is small, the uncoated marginrepresents a significant proportion of the surface. This results inoptical loses as the colours rays are not correctly redirected whenfalling on the uncoated margins.

A way to turn around the uncoated margin problem consists of coating athin glass plate whose dimensions are bigger than the prismcross-section. The plate is then glued between two prisms. The uncoatedmargin being out of the prisms cross-section, the efficiency of thedichroic filter is optimised.

The proposal inter alia allows to combine sources of different coloursand to uniformly illuminate a surface in a very in compact embodiment;this with minimal optical losses. Our proposal also show how to relaxthe dimension requirements on the colour cube.

-   -   These and further aspects will also be elucidated in the        following:

The present invention inter alia also relates to a light extractionmechanism for LED illuminators.

The optical efficiency of a LED based illumination device depends on thedesign and assembly process between the light source and the collimationoptics. We present an original optimised design as well as anassembly/manufacturing technique suitable for low cost mass-production.

The outcoupling or extraction efficiency of LEDs is generally done bythe LED manufacturer by applying a micro-structure on the chip surfaceor by giving a special shape to the encapsulating material (e.g. LEDwith epoxy lens).

The invention inter alia proposes an illumination architecture based onlight pipes. It is further proposed a system for which two opticalelements are coupled with a index matching fluid.

Some aspects of the present invention consist of:

-   -   improved light pipe efficiency by combining total internal        reflection (plain pipe) and mirror reflection (hollow pipe), and    -   conception of a assembly scheme for efficient lighting and low        cost mass-production.

The invention inter alia intends to provide solutions for the followingmain problems:

-   1) An assembly technique is proposed for low cost mass-production.-   2) The combination of a plain and hollow pipes optimises the device    illumination efficiency.

As explained above, the combination of a LED array and a pyramidal lightpipe constitutes an efficient and compact illumination system which canbe, for example, used for the illumination of micro-displays used inprojection application.

The light pipe can either be a plain light pipe for which the rays areguided by total internal reflection (TIR), or an hollow pipe whoseinternal faces are mirror coated (metallic reflection). The advantage ofplain pipes is that the redirection of ray by TIR is loss less. Theadvantage of hollow pipes is that they can be made shorter, as the rayspropagating in air have larger angles and meet the light pipe facesafter a shorter distance. On the other hand, the mirror reflection ofhollow pipe produces optical losses (typical mirror reflectivity variesbetween 92% to 98% depending on angle of incidence and mirror material).

Key parameters of such an illumination engine are:

-   1) the illumination efficiency η,-   2) the uniformity of illumination, and-   3) the compactness of the device.

The illumination efficiency η is defined as

${\eta = \frac{\phi_{opt}}{\phi_{LED}}},$where φ_(LED) is the flux emitted by an LED and φ_(opt) is the fluxprovided by the illumination engine within the limited aperture (solidangle) of the optics.

The state-of-the art of the LED light pipe illuminator suffers offollowing defaults

-   A. For plain light pipes: non optimal optical efficiency due to ray    escaping the light pipe laterally, see FIG. 29A.-   B. For plain light pipes: non optimal optical efficiency due to non    perfect index matching between the LED source and the light pipe,    see FIG. 30A.-   C. For hollow pipes: non optimal efficiency due to poor LED    extraction efficiency, see FIG. 31A.

As explained hereafter, the invention inter alia consists in avoidingthese three shortcomings, as well as in proposing an efficient assemblysolution well suited for mass production.

Ad A: Optimisation of Light Guiding

Plain light pipes suffer some losses as some ray reach the pipe facesout of the TIR condition. Note that this effect only happens for rayswith large propagation angles at the beginning of the pipe, where therays have still not been deflected.

It is proposed to overcome the loss of optical efficiency produced bythe rays escaping laterally by coating the first section of the lightpipe with a mirror. Alternatively, the plain light pipe can be pluggedinto a small hollow pipe. This way, the rays are guided by metallicreflection in the first section of the light pipe (with some minimalreflection losses), but are guided by TIR for the rest of the light pipe(without losses), see FIG. 29B.

Ad B: Optimisation of LED-to-Pipe Coupling

The light extraction efficiency of LEDs is limited by the rays which aretrapped by TIR. The TIR is the consequence of the difference of indexbetween the LED protective layer (e.g. epoxy or silicone) and the air.This can be avoided when there is an refractive index matching betweenthe LED and the light pipe. A good index matching is achieved when thereis no air gap between the LED protective layer (and the light pipe (e.g.PMMA). In practice, this is difficult to achieve due to the imperfectsurface flatness of the LED protective layer. Typical indices vary withwavelength and gave values for silicone of about 1.46, for epoxy ofabout 1.53, and for PMMA of about 1.49.

The way to avoid any the air gap between the LED and the light pipe isto insert a fluid (liquid, gel, and/or glue) whose index of refractioncorresponds to those of the pipe and the LED protection layer. Note thatdue to capillarity forces, the fluid sticks between the two parts, seeFIG. 30B.

Ad C: Optimisation of Hollow Pipes Efficiency

Due to their nature, hollow pipes cannot improve LED extractionefficiency by index matching, as LED-to-pipe coupling is not possible.To avoid this problem a small plain pipe section whose role is toincrease LED extraction efficiency is inserted to make a first rayredirection. This way the advantage of hollow pipes (shorter that plainpipe) and plain pipes (high extraction efficiency) is combined, see FIG.31B.

D: Optimisation of Assembly Process

The LED-to-pipe assembly process has to fulfil the following conditions:

-   1. Fix together the optical components (LED and light pipe)-   2. Preserve the optical properties of the device (high optical    efficiency)-   3. Be as simple as possible in order to reduce costs of the mounting    process for mass-production

The invention is based on a configuration which, as discussed, has ahigh optical efficiency. In order to keep costs as low as possible, itis proposed to use a glue as index matching material and to use thespecially shaped hollow pipe in order to fix the different componentslaterally.

Possible steps of the assembly process are:

-   1) Position the specially shaped light pipe on top/around of the LED-   2) Deposit a droplet of index matching glue-   3) Press the plain light pipe against the LED-   4) Fine tune the horizontal alignment of the light pipes and the LED-   5) Cure of the glue. Depending on the glue, the curing is either    done thermally, by infrared illumination, or by ultraviolet    illumination.

In order to maximize the light extraction efficiency, it is importantthat the volume of the glue droplet is big enough to fill the air gapvolume between the plain pipe and the LED. The air gap volume betweenthe LED and the pipe is hard to predict with precision as it depends onthe LED manufacturing process (deposition process of the epoxy). Byprecaution, the volume of the glue is chosen with some margin. Asillustrated in FIGS. 32A and 32B, the glue in excess finds its place isthe space available between the lateral faces of the LED and the hollowpipe basis.

The invention solves the following problems of the state-of-the art:

-   -   For plain light pipes: non optimal optical efficiency due to ray        escaping the light pipe laterally.    -   For plain light pipes: non optimal optical efficiency due to non        perfect index matching between the LED source and the light        pipe.    -   For hollow pipes: non optimal efficiency due to poor LED        extraction efficiency.

Moreover, the invention proposes a cheap and efficient assembly processoptimised for mass-production.

In the following the invention will be described in more detail takingreference to the accompanying Figures.

FIGS. 1, 2 illustrate a first preferred embodiment of the inventiveillumination arrangement.

FIG. 3 shows a graph which illustrates the relative emission of light asa function of the direction angle of emission for a light emittingdiode.

FIGS. 4A, 4B show details of further embodiments of the presentinvention.

FIGS. 5-9 illustrate further embodiments of the present invention formultiple colour applications.

FIGS. 10-12 illustrate some of the theoretical background.

FIG. 13 shows a first embodiment of an illumination arrangementaccording to the present invention.

FIG. 14 shows a second embodiment of an illumination arrangementaccording to the present invention.

FIG. 15 shows a third embodiment of an illumination arrangementaccording to the present invention.

FIG. 16 shows a fourth embodiment of an illumination arrangementaccording to the present invention.

FIG. 17 shows an enlargement of the configuration of the light sources,the light mixing device and the light pipe of the embodiment shown inFIG. 13.

FIG. 18 shows an alternative configuration of the light sources, thelight mixing device and the light pipe of the embodiment shown in FIG.13.

FIG. 19 shows an enlargement of a configuration of light source andlight pipe used in the embodiments of FIGS. 15 and 16.

FIG. 20 shows an enlargement of an alternative configuration of a lightsource and a corresponding light pipe used in the embodiments of FIGS.15 and 16.

FIG. 21 shows a known illumination arrangement.

FIG. 22 shows a first embodiment of a combination of dichroic filters.

FIG. 23 shows a second embodiment of a combination of dichroic filters.

FIG. 24 shows a third embodiment of a combination of dichroic filters.

FIG. 25 shows an embodiment of a color cube.

FIG. 26 shows the principle of total internal reflection.

FIG. 27 illustrates a method for coating prisms.

FIG. 28 shows an alternative method for coating prisms according to thepresent invention.

FIG. 29A, 29B are cross-sectional side views of embodiments of thepresent invention without and with a respective light collecting and/orguiding improving arrangement, respectively.

FIG. 30A, 30B are cross-sectional side views of embodiments of thepresent invention without and with a respective light collecting and/orguiding improving arrangement, respectively.

FIG. 31A, 31B are cross-sectional side views of embodiments of thepresent invention without and with a respective light collecting and/orguiding improving arrangement, respectively.

FIG. 32A, 32B are cross-sectional side views of further embodiments ofthe present invention demonstrating a mounting process with respect to arespective light collecting and/or guiding improving arrangement.

-   -   The following description is directed to preferred embodiments        of the present invention, in particular with respect to said        first solution by taking reference to FIGS. 1 to 12.

FIG. 1 demonstrates by means of a schematical and cross-sectional sideview a first preferred embodiment of the inventive illuminationarrangement 1.

The embodiment of FIG. 1 consists of a light source device 10, which isbuilt-up by a solid state light source device 30. The solid-state lightsource device 30 of the embodiment of FIG. 1 consists of an array 33 oflight emitting diodes 31. Said array 33 is formed to have a lightemitting area or light emitting aperture 30E from which primaryillumination light L1 is emitted to reach an incidence aperture 50I of alight collecting, integrating and redirecting device 20 which mayconsist as in the example of FIG. 1 of a light integrating device 50 andof a light valve device 40, the former of which is in this case formedas an integration or integrator rod 50 of a solid bulk material, forinstance glass, plastic, or the like.

Rays of primary illumination light L1 entering, said integrator rod 50via said light incidence aperture 50I are first of all refractedaccording to the Snell's law of refraction and according to a refractiveindex of the material of the integrator rod 50 being larger than therefraction index of the gap material of the gap G between saidintegrator rod 50 and the light source device 10. During the passage ofthe primary illumination light rays L1 within the material of theintegrator rod 50 said rays of light are reflected at the side walls orfaces 50 s of the integrator rod 50. Finally, after a plurality ormultiplicity of reflections at the side walls 50 s each of said receivedand multiply reflected rays of light of the primary illumination lightL1 exits from the integrator rod 50 via output aperture 50E and thenenters the light valve 40 being situated in direct proximity to thelight output aperture 50E.

After exiting said integrator rod 50 via output aperture 50E, the lightdistribution in the second gap G′ between the integrator rod 50 and thelight valve 40 is more uniform than the light distribution at the firstgap G between the light source device and the integrator rod 50.

After receiving the redirected primary illumination light RL1 therespective rays of light are allowed to pass through the light valve 40in a controllable manner and they leave the light valve 40 as secondaryillumination light L2 to enter certain projection optics 70, shown inFIG. 2, and then entering a display screen 80.

The gap G between the light source device 10 and the integrator rod 50which is shown in FIGS. 1 and 2 is of particular importance as even inthe case of an array of light emitting diodes each of said diodes 31only has a minor directive emission capability because the lightdistribution or energy distribution of emitting light waves obeys aLambertian distribution as shown in FIG. 3. FIG. 3 demonstrates thisLambertian distribution as a graph demonstrating the relative energy ofemitted light for a light emitting diode 31 as a function of theemission angle. From the distribution of FIG. 3 it can be derived, thatit is necessary to keep the gap width of the gap G between the lightsource device 10 and the integrator rod 50 as narrow as possible toincrease the integral or the amount of primary illumination light L1entering the area of incidence or incidence aperture 50I.

As can be seen from FIG. 2, the cones of acceptance of the integratorrod 50 and the displaying optics 70 may be different. Therefore, itcould be necessary to adapt said cones of acceptance. This can be donealternatively by employing fly-eye-optics as shown in FIG. 4A or morepreferably by using an integrator rod 50, having a pyramidalcross-section as shown in FIG. 4B.

FIGS. 5 and 7 to 9 demonstrate different possibilities of combiningsolid state light source devices 30 of different colours to obtain amulti-colour illumination arrangement for a multi-colour projectionsystem.

In FIG. 5 three different coloured solid state light source devices 30having e.g. light emitting diode arrays 33 are given, each of said solidstate light source devices 30 and therefore each of said light emittingdiode arrays 33 being associated with an integrator rod 50 interposedbetween said solid state light source device 30 and a light valvearrangement 40, so that for each of said light source devices 30 ofdifferent colours an arrangement similar to that shown in FIG. 1 isgiven.

To combine the three different colours of said three different solidlight source devices 30 a light mixing device 55 or colour cube 55common for each of said three arrangements is given being capable ofreceiving the respective secondary illumination light L2, to mix themup, and to allow them to pass over to the projection optics 70.

FIG. 6A to 6C show different embodiments of the light collecting,integrating and redirecting unit or device 20 in the form of differentintegrator rod arrangements each of which being adapted for an array 33of LEDs 31 or 31-1 to 31-4 as a light source device 10 and each of whichbeing optically coupled to a light valve device 40.

In FIG. 6A the light collecting, integrating and, redirecting unit ordevice 20 is formed as a plurality of more or less similar or identicalseparated and parallely arranged single pyramidal integrator rods 50-1to 50-4 each of which being uniquely assigned and coupled with itsrespective light entrance section 50I to a given LED 31-1, to 31-4,respectively. The light entrances 50I are in each case smaller than therespective light output sections 50O which are optically coupled to thelight entrance section 40I of a common light valve device 40.

In FIG. 6B the more or less similar or identical separated and parallelyarranged single pyramidal integrator rods 50-1 to 50-4 are opticallycoupled instead to the light entrance section 50I′ of a common andintegrator rod 50 the light exit 50O of which being optically coupled tothe light entrance section 40I of a common light valve device 40.

The common integrator rod 50 of the embodiment of FIG. 6B has a uniformcross section, whereas the cross section of common integrator rod 50 ofthe embodiment of FIG. 6C is non-uniform and the respective integratorrod 50 is formed pyramidal.

FIGS. 7A and 7B demonstrate two different arrangements for realizingmultiple colour illumination arrangements for multiple colour projectionsystems which differ from the embodiment of FIG. 5.

In the embodiment of FIG. 7A a solid state light source device 30 isemployed as said light source device 10 which has a LED-array 33, themembers of which, i.e. the distinct light emitting diodes 31, possessingdifferent spectral emission ranges, i.e. different colours. After thepassage of the primary illumination light L1 through the integrator rod50 at the gap G′ between the light valve 40 and the integrator rod 50,the uniform light distribution and the uniform colour distribution afterpassing the light valve 40 is then directed to the projection optics 70.

In the case of the embodiment of FIG. 7B three different coloured solidstate light source devices 30, each of which being built-up by an array33 of light emitting diodes 31 have distinct spectral ranges or colourswith respect to each other. The primary illumination light L1 of each ofsaid single solid state light source devices 30 is directed to the lightmixing device 55 which after mixing directs the output light to theintegrator rod 50 to obtain a secondary illumination light L2 at the gapG′ between the light valve 40 and the integrator rod 50 having a uniformillumination and colour distribution.

FIGS. 8A and 8B demonstrate further examples of multiple colourillumination devices. In these cases illumination light pipes 50 areused for redirecting and making uniform received amounts of primaryillumination light L1. In contrast to the embodiments discussed above,where the incidence aperture 50I at which primary illumination light L1enters the distinct integrator rod 50 and the output aperture 50E aredisposed in parallel to each other, the incidence apertures 50I andoutput apertures 50E of the embodiments of FIGS. 8A and 8B areperpendicular to each other. Therefore, primary illumination light L1emitted by solid state light source devices 30 of the embodiments ofFIGS. 8A and 8B enters the associated illumination light pipes 50 attheir base faces, whereas the redirected primary illumination light RL1exits from said illumination light pipes 50 at side faces thereof.

A difference between the embodiments of FIGS. 8A and 8B is that forobtaining a multi-colour arrangement in FIG. 8A a plurality of singlecoloured solid state light source devices 30 or LED-arrays 33 isnecessary, whereas in the embodiment of FIG. 8B multiple coloured solidstate light source devices 30 or LED-arrays 33 are provided.

Of course, in the embodiment of FIG. 8A according to the multiplicity ofsingle-coloured solid state light source devices 30 again a light mixingdevice 55 is necessary.

The embodiment of FIG. 9 demonstrates an application of the embodimentof FIG. 7B, having intermediate optics 81, 82 for adapting the cones ofacceptance between the integrator rod 50 and the light valve 40. Theintermediate optics 81, 82 consists of a lens arrangement 81 and apolarization beam splitter 82 which in combination with each othertransforms or maps the cone of acceptance of the integrator rod 50, i.e.the geometry of the redirected primary illumination light RL1, to thecone of acceptance of the light valve 40, which is in the embodiment ofFIG. 9 a reflective light valve 40 which allows the passage of secondaryillumination light L2 to the projection optics 70 upon reflection at theinterface of light valve 40.

-   -   In the following description, some general remarks about        dichroic filters and color cubes and further remarks with        respect to the present invention, in particular with respect to        said second and third solutions will be given by taking        reference to FIGS. 13 to 28.

Color combination can either be achieved by using a combination ofdichroic filters or by using a color cube.

FIG. 22 shows a combination of a first dichroic filter 10′ and a seconddichroic filter 11′. The two dichroic filters 10′, 11′ are realized asglass plates being coated by respective transmissive/reflective films16′, 17′. Each dichroic filter reflects one single color (for examplered or green) and transmits all other colors. In this example, the firstdichroic filter 10′ transmits a first color beam 12′, having for exampleblue color and reflects a second color beam 13′, of for example redcolor. The second dichroic filter 11′ transmits the first color beam 12′and the reflected second color beam 13′ and reflects a third color beam14′, of for example green color. Thus, a combined color beam 15′ isgenerated.

As can be taken from FIGS. 23 and 24, combinations of dichroic filterscan also be realized as combinations of coated prisms of for exampleglass. The combination of dichroic filters shown in FIG. 23 comprises afirst to third prism 20′ to 22′. Between the first prism 20′ and thesecond prism 21′ a first reflective/transmissive film 23′ is provided.Further, between the second prism 21′ and the third 22′ a secondreflective/transmissive film 24′ is provided. The firsttransmissive/reflective film 23′ reflects the first color beam 12′ of,for example, blue color. The second transmissive/reflective film 24′reflects the third color beam 14′ of, for example, green color andtransmits the second color beam 13′ of, for example, red color. Thereflected third color beam 14′ and the transmitted second color beam 13′pass the first transmissive/reflective filter 23′. As a consequence, acombined color beam 15′ is generated.

FIG. 24 shows another example of a combination of dichroic filters beingrealized by coated prisms. In this combination, a first to third prism25′ to 27′ are provided. Between the first prism 25′ and the secondprism 26 a first transmissive/reflective film 28′ is provided. Betweenthe second prism 26′ and the third prism 27′ a secondtransmissive/reflective film 29′ is provided. Analogous to theembodiment shown in FIG. 23, the properties of the first and secondtransmissive/reflective films are chosen in a way that the color beams12′ to 14′ are combined into a combined color beam 15′.

FIG. 25 shows an example of a color cube. A color cube 30′ comprises afirst to fourth prism 31′ to 34′. The prism surfaces forming the cubediagonals are coated in such a way that one color (for example, red orblue) is reflected, and all other colors are transmitted. Thus, a firsttransmissive/reflective film 35′ and a second transmissive/reflectivefilm 36′ are provided. As a consequence, the color beams 12′ to 14′ arecombined into a single combined color beam 15.

All examples of light mixing devices given above show possibilities torecombine different color beams into one single polychromatic outputbeam. It should be noted that these color combination schemes do notfulfill any function in order to homogenize the illumination produced bythe output beam.

Making reference to FIG. 13, a first preferred embodiment of anillumination arrangement according to the present invention will bedescribed. An illumination arrangement 40′ comprises a first to thirdlight source 41′ to 43′, a color cube 44′ showing a firsttransmissive/reflective film 45′ and a second transmissive/reflectivefilm 46′, a pyramidal light pipe 47′, and a target surface 48′ to beilluminated.

The function of the color cube 44′ is to collect and multiplex differentcolor beams generated by the light sources 41′ to 43′. For example, thefirst light source 41′ produces a blue color beam being reflected by thefirst film 45′, whereas the third light source 43′ generates a secondcolor beam of red color being reflected by the second film 46′. Thecolor beam of green color being generated by the second light source 42′passes both the first and the second film 45′, 46′. Thus, a combinedsingle output beam passes through an output surface 49′ of the colorcube 44′ and is coupled into the pyramidal light pipe 47′ through aninput surface 50′ of the pyramidal light pipe 47′. In this example, thelengths and the widths of output surfaces of the light sources 41′ to43′ are equal to the lengths and the widths of respective input surfacesof the color cube 44′ (for example the dimensions of an output surface51′ of the first light source 41′ is equal to that of an input surface52′ of the color cube 44′). However, the dimensions of the outputsurfaces of the light sources 41′ to 43′ may also be smaller than thoseof respective input surfaces of the color cube 44′.

Compared to the embodiment shown in FIG. 21, the brightness of the colorillumination of the surface 48′ is remarkably higher since an overalloutput surface of the light sources 41′ to 43′ which emits light isthree times higher than that in FIG. 21. The pyramidal light pipe isresponsible for homogenization and collimation of the illumination.

Making reference to FIG. 14, a second embodiment of an illuminationarrangement according to the present invention will be described. Inthis embodiment, the color cube 44′ of FIG. 13 is replaced by acombination of dichroic filters. An illumination arrangement 60′comprises a first to third light source 61′ to 63′, a first dichroicfilter 64′, a second dichroic filter 65′, a pyramidal light pipe 66′,and a surface 67′ to be illuminated. The first and the second dichroicfilter 64′, 65′ are realized as glass cubes having a first and a secondtransmissive/reflective film 68′ and 69′. The first film 68′ combineslight beams being generated from the second and third light source 62′,63′ into a combined light beam which enters the second dichroic filter65′ via an input surface 70′. The second dichroic filter 65′ combinessaid combined light beam and a light beam being emitted from the firstlight source 61′ by means of the second film 69′ to generate a secondcombined light beam which enters the pyramidal light pipe 66′. Theembodiment of FIG. 14 is less compact than that of FIG. 13. However, theillumination arrangement of FIG. 14 is easier to manufacture, inparticular for very compact dimensions.

Preferably, in the embodiments of FIG. 13 and 14, air gaps G areprovided between the light sources 41′ to 43′, 61′ to 63′, and the colorcube 44′/dichroic filters 64′, 65′. Preferably, air gaps are alsoprovided between the color cube 44 and the pyramidal light pipe 47′ aswell as between the second dichroic filter 65′ and the pyramidal lightpipe 66′ and between the first and the second dichroic filter 64′, 65′.The reason for this is explained in FIGS. 17 and 18.

As can be taken from FIG. 17, a first to fourth light ray 53′ to 56′ isgenerated by the light sources 41′ to 43′. Due to the air gaps G, alllight rays 53′ to 56′ are reflected by total internal reflection (TIR)by the air gaps G and are thus coupled into the pyramidal light pipe47′. Without such an air gap, only one of those light rays 53′ to 56′would have been coupled into the pyramidal light pipe 47, namely lightray 54′, as can be taken from FIG. 18. All other light rays are eitherabsorbed by the light sources 41′ to 43′ or lost. That is, without airgaps G, the rays would either be absorbed or propagate in undesireddirections producing optical losses. Some of the light rays (ray 54′ inFIG. 18) are practically unaffected by the air gaps G. Optical losses isalso the reason to employ glass cubes in FIG. 14 and not only glassplates 10′, 11′ as shown in FIG. 22. Such coated glass plates would letlight rays escape in undesired directions producing optical losses.

Making reference to FIG. 15, a third embodiment of an illuminationarrangement according to the present invention will be described. Anillumination arrangement 80′ comprises a first to third light source 81′to 83′, a color cube 84′ showing a first transmissive/reflective film89′ and a second transmissive/reflective film 90′, a first to thirdpyramidal light pipe 85′ to 87′ and a target surface 88′ to beilluminated. Preferably, in this embodiment, the light pipes 85′ to 87′and the respective light sources 81′ to 83′ are in direct mechanicalcontact with each other. This embodiment is easy to manufacture sincethe dimensions of the color cube 84′ are relatively large. To give anexample: The cube edge has a typical size of 2.0 mm to 4.00 mm in theconfigurations of FIGS. 13 and 14, whereas the color cube has thedimensions of the μ-display (e.g. 0.7″ diagonal) in the configuration ofFIG. 3. However, the present invention is not restricted to thesedimensions. Air gaps G are provided between the light pipes 85′ to 87′and the color cube 84′. Light beams being emitted by the light sources81′ to 83′ is transported by the light pipes 85′ to 87′ to the colorcube 84′, respectively, which mixes the light beams by means of thefilms 89′, 90′ to generate a combined light beam which illuminates thetarget surface 88′.

Making reference to FIG. 16, a fourth preferred embodiment of anillumination arrangement according to the present invention will bedescribed. An illumination arrangement 100′ comprises a first to thirdlight source 101′ to 103′, a color cube 104′ showing a firsttransmissive/reflective film 105′ and a second transmissive/reflectivefilm 106′, a pyramidal light pipe 107′, a target surface 108′ to beilluminated, and a first to third additional pyramidal light pipe 109′to 111′. This embodiment differs from that shown in FIG. 13 only by theadditional pyramidal light pipes 109′ to 111′. Preferably, in thisembodiment, the additional pyramidal light pipes 109′ to 111′ and therespective light sources 101′ to 103′ are in direct mechanical contactwith each other.

The embodiment shown in FIG. 16 is more compact than that of FIG. 15. Itis not as compact as that of FIG. 13, but easier to manufacture. Airgaps G are provided between the light pipes 109′ to 111′ and the colorcube 104′ and between the color cube 104′ and the pyramidal light pipe107′.

The color cube 44′ and the dichroic filters 64′, 65′ are surrounded byair gaps G in order to minimize optical losses, as already explained.However, when the source is directly in front of the light pipe, asshown in FIGS. 15 and 16, there is no need to include an air gap betweenthe source and the light pipe. This will be explained while makingreference to FIGS. 19 and 20.

Actually, when using surface mounted LEDs, the light extractionefficiency of the light source is increased when the light pipe is incontact with the LED surface (see FIG. 20). This results in ansignificant increase of the overall optical efficiency.

In FIGS. 19 and 20, a semiconductor active surface 121′ of asemiconductor 120′ is shown which emits light rays in random directions.Due to the high refractive index n1 of the semiconductor 120′ comparedto the refractive index n2 of an epoxy layer 122′, part of the rays aretrapped by total internal reflection (for example ray denoted byreference symbol 123′). In the configuration of FIG. 19, the samehappens for parts of the rays between the epoxy layer 122′ and an airgap G (see totally reflected ray denoted by reference symbol 124′).However, in the configuration shown in FIG. 20, where the epoxy layer122′ is in contact with a light pipe 126′, all the rays leaving theepoxy layer 122′ are coupled into the light pipe 126′, as the refractiveindex of the epoxy layer 122′, n2 and the refractive index of the lightpipe 126′, n3 preferably acre chosen close to each other (close to indexmatching condition).

The principle of total internal reflection is shown in FIG. 26. For aray in a high refractive index medium n1 reaching an interface 130′ witha lower refractive index medium n2, if the angle of incidence α isbigger than the critical angle αc, then the ray is totally internalreflected (TIR). Otherwise, the ray is refracted and propagates in thelow refractive index medium. The critical angle is α_(c)=sin⁻¹ (n2/n1).

When using color or dichroic cubes for the illumination configurationsproposed above, the full dichroic surface of the prisms/glass platesetc. is “used” (illuminated) by the light beams. In other words, thefull surface should be coated by the corresponding dichroic films. Inpractice, however, it may be difficult to coat the surface close to theedges for several reasons (for example because the edges are alreadycovered with undesired material).

This situation is shown in FIG. 27: A prism surface 140′ is coated by atransmissive/reflective (dichroic) film 141′ as described above.Usually, an uncoated margin 142′ is left because of restrictions in themanufacturing process. When the prism size is small, the uncoated marginrepresents a significant proportion of the prism surface 140′. Thisresults in optical losses as the color rays are not correctly redirectedwhen falling on the uncoated margin 142′.

A way to turn around the uncoated margin problem is shown in FIG. 28. Athin glass plate 145′ whose surface dimensions are bigger than thesurface dimensions of a prism cross-section of two prisms 146′, 147′ iscoated with said transmissive/reflective film 141′ and then provided(glued) between the two prisms 146′, 147′ to get a dichroic filter on aglass prism basis. The uncoated margin 142′ does then not lie within theprisms cross-section, thus the efficiency of the dichroic filter isoptimized.

-   -   The following description is directed to preferred embodiments        of the present invention, in particular with respect to said        fourth solution by taking reference to FIGS. 29A to 32B.

FIGS. 29A and 29B demonstrate by means of cross-sectional side viewsembodiments of the present invention with and without the inventivelight collecting and/or guiding improving arrangement 50A, respectively.

For collecting and integrating primary illumination light L1from a lightsource device 10 a light integrating device 50 in the form of apyramidal light pipe or integration rod 50 is provided, the latterhaving the light incidence aperture 50I and side walls 50 s. Theintegration rod 50 uses the principle of total internal reflections orTIR as is demonstrated by the collected ray C which exits the lightsource device 10 and its housing 30 c through a light exit aperture 30Eso as to enter the light integrating device 50 or integration rod 50through its light incidence aperture 50I. This happens under an anglewhich is sufficient so as to satisfy the TIR conditions for saidcollected ray C.

However ray R of FIG. 29A exits the light source device 10 through itslight exit aperture 30E under an angle such that the TIR conditionscannot be fulfilled by said ray R. Therefore said ray R is not reflectedback to the internal at the side wall 50 s of said light integrationdevice 50 but escapes from the light integration device 50. Thereforeaccording to the situation of FIG. 29A light may be lost therebydecreasing the efficiency of the arrangement of FIG. 29A.

According to a further aspect of the present invention the arrangementof FIG. 29A is modified by providing at the periphery of the side walls50 s of the light integration device 50 in the neighborhood of saidlight incidence aperture 50I reflecting means 50 m as said lightcollecting and/or guiding improving arrangement 50A or as a partthereof. According to this particular measure said ray R which is lostin the arrangement of FIG. 29A is reflected back by said reflectingmeans 50 m of said light collecting and/or guiding improving arrangement50A. Thereby, the efficiency or light efficiency of the arrangementshown in FIG. 29B is improved when compared to the arrangement of FIG.29A.

FIGS. 30A and 30B also describe illumination arrangements according tothe present invention without and with the provision of the inventivelight collecting and/or guiding improving arrangement 50 a,respectively.

In the arrangement of FIG. 30A an air gap or air gap structure G issituated between the light exit aperture 30E of the light source device10 and the light incidence aperture 50I of the light integrating device50 in the form of an pyramidal integration rod 50. Produced ray C oflight enters the internal of the light integrating device 50 after beingtransmitted by said air gap G under angle conditions which fulfill theTIR conditions of the light integrating device 50. Therefore ray Cremains collected in the internal of said light integration device 50.However ray R shown in FIG. 30A gets lost by being reflected at theinterface between the casing 30 c of the light source 10 and the air gapG according to total internal reflection. This TIR condition at theinterface between the casing 30 c and the air gap G strongly depends onthe large difference between the refraction indices of the material ofthe casing 30 c or of the light source device material and the air gap.

To overcome this difficulty in the embodiment of FIG. 30 b the air gap Gbetween the light exit aperture 30E of the light source device 10 andthe light incidence aperture 50I of the light integrating device 50 isfilled with a liquid, gel, and/or a glue or a refraction index matchingmeans 50 r as said light collecting and/or guiding improving arrangement50A or as a part thereof. Thereby, the light efficiency of theembodiment of FIG. 30B is improved when compared to the light efficiencyof the embodiment of FIG. 30A, as for instance ray R is coupled to theintegration rod 50.

FIGS. 31A and 31B demonstrate by means of cross-sectional side viewsembodiments of the inventive illumination arrangements without and withthe provision of the inventive light collecting and/or guiding improvingarrangement.

In both cases the illumination arrangement comprises as a lightintegration device 50 a hollow pipe or hollow guide tube in pyramidalform. This light integration device 50 comprises a light incidenceaperture 50 i which is positioned in the neighborhood of a light exitaperture 30E of said light source device 10. In a similar way ascompared to the embodiment of FIG. 30A a ray C may be collected by saidguide tube, whereas the ray R suffers from total internal reflection atthe interface between the light source devices' material and the air.Therefore a fraction of the light or primary illumination light L1 beingemitted from said light source device 10 via its light exit aperture 30Egets lost.

To overcome this problem a plain light pipe section 50 p as said lightcollecting and/or guiding improving arrangement 50A or as a part thereofis provided filling an end section of the hollow guide tube or hollowpipe as said integration device 50. Additionally, said, plain pipesection 50 p completely fills the end section of the hollow pipe as saidlight integration device 50 and terminates the same and its lightincidence aperture 50I. Between the end surface or light incidenceaperture 50I and said light exit aperture 50E again a refraction indexmatching means 50 r is provided so as to overcome the TIR problems atthe interface between the light exit aperture 30E of the light sourcedevice 10 and the air gap G. Thereby less primary illumination light L1is lost and additionally the light efficiency of the arrangement shownin FIG. 31B is improved over the light efficiency of the embodimentshown in FIG. 31A.

Reference Symbols 1 Illumination arrangement 10 light source device 20light collecting, integrating and redirecting device 30 solid statelight source device 30c case/case material, housing/housing material 30Elight exit aperture, light output aperture 30I light incidence aperture,light entrance aperture 30O light exit aperture, light output aperture31 solid state light source, LED 31-1 solid state light source, LED 31-2solid state light source, LED 31-3 solid state light source, LED 31-4solid state light source, LED 32 solid state light source 33 array ofsolid state light sources 40 light valve device, LCD panel 40E lightexit aperture, light output aperture 40I light incidence aperture, lightentrance aperture 40O light exit aperture, light output aperture 50light integrating device, integrator rod, light pipe 50A light couplingand/or guiding improving arrangement 50E light exit aperture, lightoutput aperture 50E′ light exit aperture, light output aperture 50Ilight incidence aperture, light entrance aperture 50I′ light incidenceaperture, light entrance aperture 50m reflecting means, reflectingmirror 50O light exit aperture, light output aperture 50p plain lightpipe section, plain pipe section 50r refraction index matching means,liquid, gel, glue 50s side wall 50-1 light integrating device,integrator rod, light pipe 50-2 light integrating device, integratorrod, light pipe 50-3 light integrating device, integrator rod, lightpipe 50-4 light integrating device, integrator rod, light pipe 55 lightmixing device, beam splitter device, colour cube device 60 displayoptics 70 projection optics 80 display, display screen 81 intermediateoptics, lens arrangement 82 intermediate optics, polarization beamsplitter 1′ illumination arrangement 2′ light source array 2′₁ firstlight source 2′₂ second light source 2′₃ third light source 2′₄ fourthlight source 2′₅ fifth light source 3′ pyramidal light pipe 4′ targetsurface 10′ first dichroic filter 11′ second dichroic filter 12′ firstcolour beam 13′ second colour beam 14′ third colour beam 15′ combinedcolour beam 16′ transmissive/reflective film 17′ transmissive/reflectivefilm 20′ first prism 21′ second prism 22′ third prism 23′ firstreflective/transmissive film 24′ second reflective/transmissive film 25′first prism 26′ second prism 27′ third prism 28′ firstreflective/transmissive film 29′ second reflective/transmissive film 31′first prism 32′ second prism 33′ third prism 34′ fourth prism 35′ firsttransmissive/relflective film 36′ second transmissive/reflective film40′ illumination arrangement 41′ first light source 42′ second lightsource 43′ third light source 44′ colour cube 45′ firsttransmissive/reflective film 46′ second transmissive/reflective film 47′pyramidal light pipe 48′ target surface 49′ output surface 50′ inputsurface 51′ output surface 52′ input surface 53′ first light ray 54′second light ray 55′ third light ray 56′ fourth light ray 60′illumination arrangement 61′ first light source 62′ second light source63′ third light source 64′ first dichroic filter 65′ second dichroicfilter 66′ pyramidal light pipe 67′ target surface 68′ firsttransmissive /reflective film 69′ second transmissive/reflective film70′ input surface 80′ illumination arrangement 81′ first light source82′ second light source 83′ third light source 84′ colour cube 85′ firstpyramidal light pipe 86′ second pyramidal light pipe 87′ third pyramidallight pipe 88′ target surface 89′ first transmissive/reflective film 90′second transmissive/ reflective film 100′ illumination arrangement 101′first light source 102′ second light source 103′ third light source 104′colour cube 105′ first transmissive/reflective film 106′ secondtransmissive/reflective film 107′ pyramidal light pipe 108′ targetsurface 109′ pyramidal light pipe 110′ pyramidal light pipe 111′pyramidal light pipe 120′ semiconductor 121′ semiconductor activesurface 122′ epoxy layer 123′ TIR ray 124′ TIR ray 126′ light pipe 130′interface 140′ prism surface 141′ first transmissive/reflective(dichroic) film 142′ uncoated margin 145′ glass plate 146′ prism 147′prism G gap structure G′ gap structure L1 primary illumination light L2secondary illumination light n1 refraction index n2 refraction index n3refraction index RL1 redirected primary illumination light

1. An illumination arrangement, comprising: a solid state light source;and a light collecting, integrating and re-directing device configuredto receive at least a part of emitted light from said solid state lightsource and to re-direct said received light, said light collecting,integrating and re-directing device includes a hollow light pipe with alight incidence aperture, which is positioned in close proximity of alight exit aperture of said solid state light source, and an end sectionof said hollow light pipe in close proximity of said light incidenceaperture is filled with a plain light pipe.
 2. The illuminationarrangement according to claim 1, wherein between said light incidenceaperture of said light collecting, integrating and re-directing deviceand said light exit aperture of said solid state light source refractionindex matching means are provided filing a gap or a gap structurebetween said light incidence aperture of said light collecting,integrating and re-directing device and said light exit aperture of saidlight source device.
 3. The illumination arrangement according to claim2, wherein said refraction index matching means is a liquid, gel, and/ora glue.
 4. A method for manufacturing an illumination arrangementcomprising: positioning a hollow light pipe on top/around a solid statelight source; depositing a droplet of index matching glue on the solidstate light source; inserting a plain light pipe into the hollow lightpipe; pressing the plain light pipe against the solid state lightsource; and curing the glue.
 5. The method according to claim 4, furthercomprising: fine tuning a horizontal alignment of the light pipes andthe solid state light source before curing the glue.